The following patent documents are believed to represent the current state of the art:
U.S. Pat. Nos. 3,585,996; 4,821,731; 5,042,486; 5,078,714; 5,099,845; 5,370,640; 5,375,596; 5,386,828; 5,425,367; 5,498,239; 5,526,820; 5,592,939; 5,622,169; 5,727,552; 5,727,553; 5,749,835; 5,755,668; 5,843,076; 5,860,938; 5,899,860; 5,904,657; 5,983,126; 6,038,468; 6,052,610; 6,073,043; 6,226,546; 6,230,042; 6,298,261; 6,304,769; 6,618,612; 6,690,963; 6,711,429; 6,741,883 and 7,197,354;
U.S. Published Patent Application Nos. 2002/0165448; 2004/0006268; 2004/0097804; 2004/0019447; 2004/0147837; 2005/0256398; 2006/0030833; 2006/0116572; 2006/0247522; 2006/0281990; 2007/0016006; 2007/0032723; 2007/0135713; 2007/0197899; 2007/0232896; 2007/0276216; 2007/0282197; 2007/0287909 and 2008/0009711;
Published PCT Patent Application Nos. WO 00/068637A1 and WO 07/015180A1; and
European Patent No. EP 1,322,377.